This invention relates to improved cation exchange membranes, to methods for their production, and a process for electrolysis of sodium chloride by using the same. More particularly, this invention relates to an improved electrolysis process by use of a cation exchange membrane comprising a perfluorocarbon polymer containing specific carboxylic acid groups of the formula ##STR3## (wherein M is a hydrogen, metal or ammonium group and R.sub.f is a group containing perfluoroalkyl having 1 to 5 carbon atoms).
It has been known to the art to obtain a cation exchange membrane of a perfluorocarbon polymer containing pendant sulfonic acid groups by saponification of a membrane prepared from a copolymer of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonyl fluoride. This known perfluoroacarbon type cation exchange membrane containing only sulfonic acid groups, however, has the disadvantage that the membrane, when used in the electrolysis of an aqueous solution of an alkali metal halide, tends to permit penetration therethrough of hydroxyl ions back migrating from the cathode compartment because of the high hydrophilicity of the sulfonic acid group. As a result, the current efficiency during electrolysis is low. This is a special problem when the electrolysis is used for the production of an aqueous solution of caustic soda at concentrations of more than 20 percent. In this reaction, the current efficiency is so low that the process is economically disadvantageous compared with electrolysis of aqueous solutions of sodium chloride by the conventional mercury process or diaphragm process.
The disadvantage of such low current efficiency can be alleviated by lowering the exchange capacity of the sulfonic acid group to less than 0.7 milliequivalent per gram of the H form dry resin. Such lowering, however, results in a serious decrease in the electroconductivity of the membrane and a proportional increase in the power consumption. This solution, therefore, is not without its economic difficulties.
U.S. Pat. No. 3,909,378 discloses composite cation exchange membranes containing sulfonic acid moieties as the ion exchange group and comprising two polymers with different equivalent weights (EW), that is the number of grams of polymer containing one equivalent of ion exchange functional group. When such membranes are utilized in the electrolysis of aqueous solutions of sodium chloride, high current efficiencies are obtained by effecting the electrolysis with the higher EW polymer side of the composite membrane facing the cathode. For high current efficiency coupled with low power consumption, the value of EW of the higher EW polymer must be increased and the thickness decreased as much as possible. It is, however, extremely difficult to produce a composite cation exchange membrane having a current efficiency of not less than 90 percent by use of membranes containing only sulfonic acid groups.
In U.S. Pat. No. 3,784,399, German Patent OLS No. 2,437,395 (U.S. patent application Ser. No. 406,361 filed Oct. 15, 1973) and German Patent OLS No. 2,447,540 (U.S. patent application Ser. No. 425,079 filed Dec. 17, 1973), there are suggested cation exchange membranes wherein the cathode side surface layers of fluorocarbon cation exchange membranes contain sulfonamide groups, salts thereof or N-mono-substituted sulfonamide groups. These membranes, however, are deficient in electrochemical and chemical stabilities.
U.S. patent application Ser. No. 701,515 (Canadian Patent Application No. 256,437) discloses a cation exchange membrane containing carboxylic acid groups of the formula --OCF.sub.2 COOM in the fluorocarbon cation exchange membrane having sulfonic acid groups. This cation exchange membrane is excellent in electrochemical properties and chemical stability, as compared with conventional cation exchange membranes. Furthermore, when used as a diaphragm for electrolysis of an aqueous sodium chloride solution, it exhibits high current efficiency with a small amount of electric consumption. From the practical standpoint, however, this membrane is not yet satisfactory in current efficiency which is desired to be high with stability during prolonged electrolysis, electric resistance of the membrane, etc.
The object of the present invention is to provide an improved electrolysis process using a cation exchange membrane high in current efficiency which is free from the drawbacks of the membrane of the prior art as mentioned above.